Egg yolk is used in the preparation of foodstuffs by virtue of its excellent properties such as emulsification and heat-gelation
It was known that the functional properties of egg yolk for use in the preparation of foodstuffs depend particularly upon lipoprotein which is a higher structure containing about 15% w/w of proteins and 10% w/w of phospholipids. Phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) are known to be prevalent in the phospholipids of egg yolk; the following Table 1 indicates typical phospholipids present in hen egg yolk:
TABLE 1 ______________________________________ Phosphatidyl choline (PC) 84.3 (mol %) Phosphatidyl ethanolamine (PE) 11.9 Sphingomyelin 1.9 Lysophosphatidyl choline 1.9 ______________________________________ (Source: Food Chemical, Vol. 1, No. 7, page 76, 1985)
It is also known to modify the properties of egg yolk by treatment with a phospholipase. For example, JP-A-84785/76 corresponding to U.S. Pat. No. 4,034,124 discloses an emulsion containing a phospholipoprotein which has been modified by the action phospholipase A (PL-A), which is prepared by a process comprising subjecting a phospholipoprotein to the action of PL-A at a temperature of 60.degree.-90.degree. C. to give a specified ratio of conversion. In this case, the material containing phospholipoprotein is preferably whole egg or egg yolk. The resultant emulsions exhibit increased heat-stability.
This patent also describes the use of phospholipase D (PL-D), commercially available from Sigma, U.S.A. It appears that the particular mentioned PL-D does not originate from a microorganism but is of plant origin, since no PL-D originating from microorganism (for instance, a microorganism of the genus Streptomyces) was ever mentioned in the Sigma catalogues published earlier than 1986.
Japanese Patent Publication 34937/72 discloses that the heat-coagulation of egg albumen and yolk, contained in foodstuffs which are prepared by processing hens eggs, can be prevented by treating a solution of whole egg or egg yolk with an enzyme at a pH of 3.5-4.5 until the heat-coagulating properties disappear. The enzymes which may be used are obtained from molds such as Aspergillus and Rhizopus. However the enzyme-treated egg is obviously not preferred for use in the preparation of certain foodstuffs which require the heat-gelation properties.
In JP-A-51853/83 corresponding to U.S. Pat. No. 4,478,866, lysophosphatidic acid (LPA) was disclosed as being particularly advantageous as an emulsifier for use in farinaceous foodstuffs such as doughs and that such an emulsifier may be partly comprised of lyso-phosphatidic acid (LPA) which may be prepared by treating phospholipids with PL-D and PL-A. This patent discloses the use of PL-D derived from plants but not the use of PL-D derived from microorganisms.
Journal of the Science of Food and Agriculture, 32, 451-458, 1981 discloses that a mayonnaise which is considered as a n acidic oil-in-water type emulsion, having good heat-stability, was prepared by using an egg yolk treated with phospholipase A (PL-A).
In this case, the enzyme used was obtained from porcine pancrease.
U.S. Pat. No. 4,034,124 discloses that oil-in-water emulsions having improved heat-stability can be prepared when a phospholipo-protein which has been modified by the action of PL-A is incorporated during some stage of the emulsion preparation. This patent also discloses that phospholipases other than PL-A do not bring about the desired characteristics of the patented product. A suitable source of PL-A for this patent is pancreatin which is preferably heat-treated, preferably under acidic conditions.
Thus, it has hitherto been difficult to obtain whole egg or egg yolk, which exhibits both excellent heat-gelation properties and emulsifying properties, by the use of PL-D of plant origin. Also whole egg or egg yolk prepared by the use of PL-A exhibits decreased heat-gelation properties.
The provision of egg yolks having improved properties, for example, heat-gelation ability such as gel strength and emulsifying properties, which are particularly advantageous for use in the preparation of certain foodstuffs, has continuously been desired.